A neutron chopper according to the present invention includes a housing which internally forms a sealed space, the housing having window portions through which neutrons pass, a fixed shaft which is fixed inside the housing, a rotor which is rotatably supported by the fixed shaft, the rotor provided with a blocking portion which can block neutrons passing through the housing, and a motor which is provided inside the housing for rotating the rotor of the neutron chopper, where a stator of the motor is fixed to the fixed shaft, and a rotor of the motor receives a rotating force from the stator around the fixed shaft, and is fixed to the rotor of the neutron chopper. The neutron chopper is formed with small size, and neutron guides are easily disposed closely, consequently vacuum leak is hardly occurred in the neutron chopper.